1. Technical Field
The present invention relates to an image forming apparatus, a control apparatus, a computer readable medium storing a program and a control method.
2. Related Art
Generally, in a color image forming apparatus such as a color copy machine or a color printer, an image is formed on a paper sheet through the processes of: sequentially forming and superposing color toner images on, for example, an intermediate transfer member; and collectively transferring the color toner images from the intermediate transfer member onto the paper sheet. In this event, the color image forming apparatus sometimes performs the process of collectively transferring color toner images on a paper sheet concurrently with the process of forming, on the intermediate transfer member, the toner images for the paper sheet or toner images for an adjacent paper sheet to the paper sheet. In addition, the color image forming apparatus sometimes performs the process of collectively transferring color toner images on a paper sheet concurrently with a fixing process.
Meanwhile, to prevent a collective transfer unit and any of paper-sheet transporting members placed around the collective transfer unit from pulling a paper sheet in opposite directions, a paper-sheet transport speed of each of these paper-sheet transporting members is generally set such that the paper-sheet transporting member located more upstream has a higher paper-sheet transport speed. For example, a paper-sheet transport speed of transport rolls located upstream to the collective transfer unit is set higher than a speed of the intermediate transfer member. In addition, a paper-sheet transport speed of a fixing device located downstream to the collective transfer unit is set lower than the speed of the intermediate transfer member. As a result, when a paper sheet transported by the transport rolls enters the collective transfer unit, the paper sheet applies an additional force in its transport direction on the intermediate transfer member, and thus a load on the intermediate transfer member changes. Similarly, when a paper sheet enters the fixing unit located downstream to the collective transfer unit, the load on the intermediate transfer member changes. Hence, when a paper sheet enters the collective transfer unit or the fixing unit, the transport speed of the intermediate transfer member is likely to change, and this change affects toner image formation on the intermediate transfer member.
Accordingly, around the times such as when the collective transfer starts and when a paper sheet enters the fixing unit, color toner images might be displaced on the intermediate transfer member, resulting in a color shift or the like in the obtained image.